Loss of drilling fluids (lost circulation) is a technical problem in petroleum engineering which is ubiquitous in the procedure of oil drilling and has not been completely solved yet. Especially, serious leakage formations, such as lower pressure formation, fracture formation and cavernous formation are often encountered during the drilling constructions of deep wells or drilling constructions in complex formations. According to incomplete statistics, China spends RMB 200 million each year on treating lost circulation. Consequently, developing a convenient and effective leakage stoppage technology used in oil drilling, or providing an efficient and practical lost circulation additive will bring about good economic benefits and social benefits.
During the drilling procedure, when a drilling rig collides with a high-permeability formation especially a formation having a developing fracture or a larger cavern, more serious problem of lost circulation often happens. Currently, after lost circulation in the procedure of oil drilling occurs, measures of stopping circulation of the drilling fluid, tripping out the drilling rig, and performing leaking stoppage constructions under static conditions are typically taken. Specifically, bridging granular materials or chemical lost-circulation materials like cement slurry and urea formaldehyde resin are generally used for stopping lost circulation. With respect to lost circulation caused by a high-permeability formation, the technical requirements of drilling construction can generally be met by the use of bridging materials. However, generally, in the case of severe lost circulation caused by the situations that a drill rig encounters a pressure-sensitive formation, a big fracture, or a cave and the like, the first-time success rates of leaking stoppage with bridging materials or chemical lost-circulation materials in the prior art are low, along with the frequent occurrence of repeated leaking stoppage. For example, when using cement slurry for leaking stoppage, a cement slurry is mixed and diluted with formation water (or a drilling fluid), which often leads to the phenomenon that the cement slurry fails to concrete.
At present, lost circulation additives for well drilling mainly fall into several categories such as solid particles, fiber, inorganic gel, and anionic polymer gel and so on. CN100556979C discloses a self-cementing lost circulation additive for well drilling and preparation method thereof. Such lost circulation additive stop the lost circulation of well drilling by using inorganic particles and inorganic gel as the lost circulation additive, specifically using bauxite, limestone, and gypsum as raw materials. CN102127403B discloses a lost circulation additive for welling drilling which is prepared by uniformly mixing walnut shell powder, sawmilling powder, cotton seeds, husk powder, active blocking agent and elastic rubber particles, etc. The formulation types thereof are varied. CN101955763B discloses a high-pressure resisting lost circulation additive, which is comprised of soybean granules, walnut shell powder, vermiculite, peanut shell powder, cement and a flocculating agent (an anionic polymer) used for a drilling fluid. This lost circulation additive performs leaking stoppage by utilizing bridging between a chemical agent and a particulate matter, the construction procedure of which is more complicated. CN101586023B discloses a pre-crosslinked gel lost circulation additive for well drilling and preparation method thereof, comprising the steps of mixing acrylamide, acrylic acid, cationic monomer, cross-linking agent and an initiator above the ground, and performing cross-linking polymerization under protection of nitrogen, to give an indiscerptible, and swelling pre-crosslinked gel lost circulation additive having the expandable characteristics, wherein the used cationic monomer is dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, or acryloyloxyethyl trimethyl ammonium chloride. After this lost circulation additive is injected below ground, plugging is performed depending on continuous expansion of polymer particles. This lost circulation additive is capable of playing a favourable effect of leakage stoppage by means of swelling characteristics, but the preparation thereof is relatively complicated, for it not only needs to be polymerized under the protection of nitrogen above the ground before use, but also has continuous swelling action, thus restricting the time of use to some extent.
In addition, resin chemical agents such as urea formaldehyde, phenolic aldehyde and the like can also be used for plugging in field constructions. There are the following problems in these constructions. First, the cost of urea formaldehyde is relatively high. The construction is full of difficulty. When being contaminated by a drilling fluid, or when the environment temperature changes, concretion of the lost circulation additive of urea formaldehyde will be difficult, which leads to a low success rate of plugging.
Therefore, the present disclosure aims to develop a cationic copolymer and a lost circulation additive formed by the cationic copolymer, and to provide an effective new method for drilling plugging.
The cationic copolymer is a binary/multi copolymer containing a cationic unit and a non-cationic unit, and can be prepared through two methods substantially, including cationic modification of a polymer and copolymerization of a non-cationic monomer with a cationic monomer. Taking preparation of cationic polyacrylamide for example, cationic modification thereof comprises introducing an amine molecule to a polyacrylamide molecule by means of a Mannich reaction, followed by quaternarization to produce a cationic copolymer. The amines commonly used for such modification are dimethylamine, diethylamine, trimethylamine, and the like. Although the technical procedure is simple and easy to operate, through cationic modification, only products of a low cationic degree can be obtained, and insoluable polymers can be generated easily in the course of production, which will affect performance of the product. At present, copolymerization of a non-cationic monomer and a cationic monomer is generally used in preparing cationic copolymers.
In order to obtain a cationic copolymer of a specific use, copolymerization of a non-cationic monomer and a cationic monomer is generally used. Currently, cationic polyacrylamide and derivatives thereof are most widely prepared among all cationic copolymers. Based on the reaction mechanism that copolymerization of monomers is triggered by free radicals, the preparation method thereof comprises selecting comonomers, and determining an optimal copolymerization reaction system and procedure conditions of polymerization, so as to prepare the desired polymer. The commonly used cationic monomers in synthesis are diallyl dimethyl ammonium chloride, methacryloyloxyethyltrimethyl ammonium chloride, acryloyloxyethyl trimethyl ammonium chloride and the like.
For example, polydiallyl dimethyl ammonium chloride which is prepared by the copolymerization of diallyl dimethyl ammonium chloride and acrylamide or acrylate is a water-soluable cationic copolymer, often used as a flocculant and has a number advantages like high positive charge density, good water solubility, non-toxic, as well as low costs. It is widely used in the fields of sewage treatment in oil exploitation, papermaking, and textile dying, etc. As the relative molecular mass of the homopolymer of diallyl dimethyl ammonium chloride is low, copolymerization between diallyl dimethyl ammonium chloride and a non-cationic monomer is generally used.
By adopting inverse emulsion copolymerization, researchers have prepared polydimethyl diallyl ammonium chloride/acrylamide cationic flocculants, which has a number of advantages such as high relative molecular mass of polymers, good water-solubility, a lower content of remaining monomer. There are researchers who have used acrylamide and acryloyloxyethyl trimethyl ammonium chloride as monomers to perform aqueous adiabatic polymerization, and thus prepare a cationic polyacrylamide with relative molecular mass greater than 10,000,000. The flocculation and settling test of the product on the grey water of a paper-making factory indicates that its performance overmatches that of commonly used anionic flocculants. In treating washing and bleaching effluent, cationic flocculants has the effects of both charge neutralization, and adsorption and bridging, and the increase of the relative molecular mass thereof can improve adsorption and bridging, increase articulate size, and thus make for the flocculation and settling.
In the prior art, a chemical lost circulation additive obtained from a cationic copolymer has some effects in plugging when being used in field practice, hut also shows low plugging efficiency, weak stoppage intensity, and a short valid period of plugging in complex formations. The main reasons as to why the success rate of the conventional plugging technology in drilling engineering is low are as follows. The cationic degree of the cationic lost circulation additive is relatively low, and the synergistic action between the cationic lost circulation additive and the drilling fluid is relatively weak, and thus the variation in viscosity is small after the cationic lost circulation additive is mixed with the drilling fluid, thereby failing to significantly improve the flow resistance of the lost circulation additive. This prevents the lost circulation additive from remaining and curing effectively in a target zone such as fissure, hole and the like, thus leading to poor plugging effects. When the existing technique of leakage stoppage in well drilling is used, in the event of serious lost circulation, from hundreds to thousands of cubic meters of a lost circulation additive is typically used for construction, which not only increases costs of drilling well, but also leads to serious damage to the production capacity of oil-gas well. Therefore, developing a new cationic copolymer with wide adaptability, fast blocking speed, and high intensity, as well as a new chemical leakage stoppage, is a vital research direction in the field of oil drilling.